The following abbreviations used in the specification and/or the drawings are defined as follows:
3GPP third generation partnership project
CSG closed subscriber group
DL downlink (network towards UE)
eNodeB base station of a LTE/LTE-A system
E-UTRAN evolved universal terrestrial radio access network
ID identifier
LTE long term evolution (of the E-UTRAN system)
MAC medium access control
MME mobility management entity
MTC machine type communication
NAS non-access stratum
PLMN public land mobile network
S-GW serving gateway
SIB1 System Information Block Type1
SIB2 System Information Block Type2
SCH shared channel
RRC radio resource control
UE user equipment
UL uplink (UE towards network)
In the E-UTRAN system there are conventional network access nodes/cells termed eNodeBs which serve all users, and also subscriber group CSG cells such as home eNodeBs which are available for traffic (voice and/or data) to only those subscribers registered with the CSG and possibly also certain allowed guests. Any given CSG may include a group of cells (such as a corporate or university campus) or a single cell. CSGs may allow traffic access for non-subscribers for emergency calls or routine, non-emergency access.
These different types of cells or access nodes may be termed more generally as public access nodes/public cells and private access nodes/private cells. Other wireless systems (GERAN, GSM, UTRAN, WCDMA) have either implemented or are considering implementing similar such private networks as more functionality is shifted from higher in the radio access network toward the base stations/access nodes.
There are two types of conventional closed subscriber group (CSG) implementations, namely CSG-aware user equipment (UE) and non-CSG-aware UE. In the CSG-aware UE implementation, CSG subscription data is maintained at the home location register and is retrieved by mobility management entities, such as MME, MSC/VLR, SGSN, as part of UE's subscription profile. In the LTE system the CSG-aware UE maintains a list, termed a CSG whitelist or CSG allowed list, which lists the identities of the CSG cells for which that particular UE has access rights. 3GPP TS 23.401 states that the UE is to keep this CSG whitelist, which can be either the “allowed CSG list” or the “operator CSG list” contained in the so-called UE context which is provided by the UE's home network to any other networks the UE is transiting. Each CSG list has the form of a list of CSG IDs and the associated PLMNs. Each CSG ID reliably identifies a CSG, which as above might include a single CSG cell or multiple CSG cells. Prior to requesting access to a CSG cell, UEs verify whether its CSG ID is in their white list, and only request access if it is. Users can override this automatic CSG cell screening by manually selecting a CSG cell that is not in their UE's allowed list.
When a CSG-aware UE accesses such a CSG cell, the MME/SGSN/MSC/VLR checks that the CSG ID of the CSG cell corresponds to a CSG ID in the CSG subscription data, and that the relevant validity criteria are met.
As regards the non-CSG-aware UE implementation, CSG subscription data is maintained by CSG operation, administration and maintenance (OAM) function.
Access to CSG cells may be further controlled on the basis of service operator assigned classes. There are 15 such classes (0-9 assigned to low priority users (so-called “Ordinary UEs”) and 11-15 assigned to high priority users, such as emergency services), and the entire population of UEs are allocated to a service access class. If any given CSG cell allows, for example, service access class 0 to camp on its cell, then if there are 100,000 users of this class, this gives rise to a significant number of users that may potentially require access to the given CSG cell.
Ordinary UEs are barred from accessing a given CSG cell based on RRC configuration parameters for access barring that are broadcasted on SIB2 (i.e. access class barred, barring rates, barring probability). The broadcast of enhanced access barring by the RAN has been proposed in TR 23.888 v11.1.2 in order to restrict a specific congesting MTC Group/APN from attempting access to a CSG cell, or to prevent all MTC Devices, low-priority MTC Devices, and/or MTC Devices of a PLMN type from attempting access. The RRC and/or NAS rejection back-off times and MTC access barring randomization can successfully prevent the rejected/barred MTC Devices from almost simultaneously initiating access attempts after the congestion scenario or the overload scenario have subsided.
The aforementioned access barring mechanism for Ordinary UEs is based on their access class, and, as alluded to above, since there are a significantly larger number of low priority users than high priority users, it applies to relatively large mobile populations. While such barring mechanisms are suited to special scenarios where UEs may be prevented from making access attempts in case of states of emergency, they are not suitable as a means to prevent low-priority UEs from making access attempts to a eNodeB based on congestion or overload considerations.
Embodiments are directed towards providing an improved method of controlling access to private cells such as CSG cells.